A difficulty for practical applications of intra-cavity frequency-doubling laser sources is that in operation the intra-cavity frequency-doubling laser sources need to be controlled in narrow temperature ranges. The tight temperature control is necessary in order to circumvent a technical problem generally known as the “green noise”. The increased applications of the intra-cavity frequency-doubling laser sources have generated an urgent need to remove or relax the temperature-control requirement. Since such laser sources have advantages of compact size, high-energy efficiency, stable frequency, high-quality light beam, low thermal effect, and long lifetime, the lasers source can be readily applied to biomedical fields and display devices. The diode pumped, intra-cavity frequency-doubling lasers can be found in much wider range of applications the laser sources with low noise level are available without the need for tight temperature control. The green noise is generated from the coupling between the longitudinal modes through cross saturation of the gain and sum-frequency mixing. Many attempts have been made to overcome the “green noise” problem and the related temperature-control requirement.
One way to overcome the above described problem is to create a laser system that operates in a single frequency. Such a system can provide an operation condition that could minimize or even totally eliminate the problems of green noise. A drawback of the single frequency operation is low energy efficiency, high cost, and much tighter operation conditions. Single frequency laser operation is thus impractical due to these intrinsic drawbacks.
Another solution to eliminate the green noises is to deal with the root cause of noise generation based on detail investigations of the characteristics of the optical interactions in the processes of optical resonance and frequency doubling taking place in the intra-cavity. In general, a diode pumped, multimode intra-cavity frequency-doubling laser with low noise as available now typically includes a birefringent gain medium, specially orientated birefringent nonlinear crystal. If the optical thickness and orientation of both gain medium and nonlinear crystal meet certain conditions, the green noise in second harmonic output laser radiation is compressed. However, since the length and the refractive index of nonlinear crystal is strongly temperature-dependent, the conditions for low noise operations are easily broken with variation of environmental temperature. Normally, such a laser requires expensive, high precision temperature controller to keep operation temperature of the laser in around 0.1° C.
Therefore, a need still exists in the art of manufacturing and designing the laser sources to provide configurations and methods to remove such stringent temperature control limitations.